As known, analog nonvolatile memory cells are currently read by a reading device comprising a sense amplifier and an analog/digital (A/D) converter. In detail, the sense amplifier is connected to the memory cell to be read and generates an output voltage having an amplitude correlated to the value of the current flowing in the memory cell during reading, and thus correlated to the threshold voltage of the memory cell. The A/D converter receives at the input the output voltage generated by the sense amplifier and supplies at the output a binary word associated to the threshold voltage of the memory cell.
Specifically, the A/D converter comprises a plurality of comparator circuits receiving at a first input the output voltage of the sense amplifier, and at a second input different preset reference values, typically defined by a resistive divider receiving at the input a first and a second reference voltages defining the conversion range and the value of the least significant bit of the output binary word.
The reference voltages for the A/D converter are generated inside the chip accommodating the A/D converter, for example by temperature compensated regulator circuits, or are supplied from outside. In both cases, however, the reference voltages supplied to the A/D converter are completely uncorrelated with the variations in time in the output voltage of the sense amplifier, which are mainly due to the variation in the threshold voltage of the memory cell. In certain cases, this may cause reading errors of the memory cell.